Near field communications (NFC) is a set of protocols that enable, for example, smartphones and other devices to establish radio communications with each other by either touching the devices together, or bringing them into close proximity, say to a distance of typically 10 cm or less. NFC always involves an initiator device and a target device; the initiator device actively generates a radio frequency (RF) field that can power a passive target device. This enables NFC target devices to take very simple form factors, such as tags, stickers, key fobs, or cards that do not require relatively large power supplies. NFC peer-to-peer communication is possible, provided both devices are powered.
Thus, in order to support near field communications, NFC devices fall into two main areas: NFC tags and NFC readers/writers. NFC tags often securely store personal contacts, such as debit and credit card information, loyalty program data, PINs, and networking contacts, among other information. NFC tags contain data and are typically read-only, but in some instances may be re-writable. NFC tags can be custom-encoded by their manufacturers, or they can be configured in accordance with specifications provided by a relevant industry association.
NFC readers/writers are typically NFC-enabled devices configured to read information stored on inexpensive NFC tags embedded in, say, labels or smart posters. Both NFC tags and NFC readers/writers are known to have interchangeable functionality and similar (or the same) components and circuits. As such, an NFC device may often function as either a NFC tag or a NFC reader/writer.
One known problem in the provision of NFC-capable devices is the ability of such devices being able to equalize high-data rate signals (e.g. data rates of greater than 848 kbps), as lower data rates do not require equalization. For example, today's RF-ID systems use a 1-bit-per-symbol amplitude-shift-keying (ASK) modulation with maximum potential data rate limited to 6.78 Mbps. However, some companies are proposing schemes to overcome these limitations and reach data rates up to 20.34 Mbps and more. Furthermore, due to the high coupling requirements of a NFC system, and the effects on the coupling due to varying channel characteristics, high-quality (‘Q’) matching networks are required. If high-quality (‘Q’) matching networks are used, inter-symbol interference (ISI) of the transferred data may degrade the bit error rate (BER) performance of the NFC link down to more than 0.01%.
Additionally, it is problematic to equalize the high-data rate signals under normal operating constraints, such as: low computational complexity, minimum convergence time and ensuring a high guarantee of convergence. A typical NFC link will also have to cope with different communication channel types, with no loopback schemes or training sequences being available to be used.
FIG. 1 illustrates a known block diagram of a NFC link showing both an uplink communication path 100 and a downlink communication path 150 and the types of signals encountered. In the first uplink communication path 100 load (amplitude) modulation 122, 124 is employed by the NFC devices. A NFC reader 104 instigates a sinusoidal waveform 110 into a radiated field 106, which is looped back to the NFC reader 104 circuitry. A tag 102 varies the impedance in the radiated field 106 by switching ON/OFF a resistor/capacitor (load) 108. In this manner, binary data is encoded onto the sinusoidal waveform 110 as impedance variation and results in modulation of the sinusoidal waveform 120 being returned to the NFC reader 104. In such NFC systems the modulation index can be <1%.
In the second downlink communication path 150, waveform modulation is employed. A NFC reader 154 instigates a sinusoidal waveform 172 into a radiated field 156 to be received by a tag 152. The data is directly mixed with carrier, or multiplied with a sub-carrier before mixing with the carrier. In this manner, encoded binary data may be recovered. In such NFC architectures a minimum modulation index can be of the order of 8%.
Furthermore, in a number of applications, where the communication channel does not exhibit flat fading, the load modulation NFC devices may create inter-symbol interference (ISI) when used with high symbol rates. The NFC channel characteristics are shown in the graph 180, which illustrates two poles in the NFC frequency response. A first pole 182 is determined by NFC reader 104 circuitry (typically with a 1-2 MHz variation as shown), with a second pole 184 determined by the NFC tag 102 (with a 3-7 MHz variation (not shown)).
With such load modulation and waveform modulation techniques, a timing reference and accurate processing of the modulated signals is required in order to correctly recover and demodulate the data. In typical communication techniques, a timing reference can be provided by using a reliable training sequence at the transmitter for the receiver to lock on to.
Alternatively, or for high data rate applications, an equalization technique may be used. As no loopback path may exist in an NFC system, because such systems need to be inherently of low complexity, such an equalization path needs to use blind estimation. Blind estimation techniques do not require a training sequence, as known to those skilled in the art. However, blind estimation techniques introduce problems, such as an increased convergence time due to requiring thousands of bits to achieve convergence, which increases the system's buffering requirements and latency.
U.S. 2013/0064271 A1, by R. C. H. Van de Beek, M. Ciacci, and titled “Adaptive equalizer and/or antenna tuning”, describes a loopback training approach whereby a loopback signal is used as training for adaptive pre-equalization. Notably, in order to provide a reliable estimation of the channel in the loopback path, which is used as training for an adaptive pre-equalizer, each channel has its own equalizer. However, in this loopback training approach, the loopback channel is different from the signal channel, thereby introducing errors and inaccuracies. Additionally, loopback training approach requires more than one mixer, thereby adding to the cost and complexity.
Thus, there is a general need for improved concepts to equalize data, and particularly for equalizing data in very high bit rate (VHBR) near field communications (NFC).